In recent years, the development of photovoltaic power plants tends to be large-scale and centralized. The capacity of China's single photovoltaic power plants has grown from 10 MW in 2008 to 100 MW in 2012. In 2018, the scale of photovoltaic power plants in Golmud has reached 2 GW. However, these power plants are mostly built in remote areas such as deserts and plateaus, which are far from the load center and at the end of the grid, making power transmission difficult.
A photovoltaic DC boost collection and grid connection technique is a new type of photovoltaic power generation technique. By a photovoltaic high-voltage DC grid-connected converter, the low-voltage DC photovoltaic array is boosted into a medium-voltage DC or high-voltage DC to connect to a DC grid. The DC boost grid connection can solve problems such as alternating current grid-connected harmonic resonance and reactive power loss in large-scale photovoltaic generation bases, which is suitable for long-distance power collection of large-scale photovoltaic power generation bases.
In photovoltaic DC boost collection and grid connection systems, employing a connection method where input ends of a plurality of series-type photovoltaic DC boost converters are connected to independent photovoltaic arrays and output ends of the plurality of series-type photovoltaic DC boost converters are connected in series, the systems can realize secondary boost without an additional boost converter, which has low cost, high efficiency, and has obvious technical advantages. In series systems, the series-type photovoltaic DC converter not only needs to have a wide input voltage range to meet requirements of the photovoltaic array, but also needs to have a wide output voltage range, otherwise the maximum power point voltage of the photovoltaic array will not match the output voltage of the converter under some working conditions, resulting in power curtailment.
In order to obtain the wide output voltage range of the converter, some techniques such as the method where a boost circuit is added in addition to a DC isolated conversion circuit to improve the output voltage range of the converter is used[1][2] However, the method increases a circuit conversion link, has a complicated topology, increases the cost and decreases the efficiency. Moreover, the DC isolation conversion link in the method does not use a boost circuit, and the boost ratio is low.
In summary, in the topology scheme of the prior art, the structure is complicated, numerous transformation links are required, the practicability is poor, and the boost ratio is low.
The following literature is the technical background information related to the present disclosure.    [1] Christian A. Rojas, Samir Kouro, Marcelo A. Perez, Javier Echeverria, DC-DC MMC for HVdc Grid Interface of Utility-Scale Photovoltaic Conversion Systems, 201801.    [2] Yushan Liu, Haitham Abu-Rub, Baoming Ge, Front-End Isolated Quasi-Z-Source DC-DC Converter Modules in Series for High-Power Photovoltaic Systems—Part I: Configuration, Operation, and Evaluation, 201701.